Considerable evidence suggests that inputs from the vestibular system have direct and powerful influences on sympathetic outflow, as well as blood pressure. Electrical stimulation of the vestibular nerve alters activity recorded from sympathetic nerves innervating many body regions; these effects can be accompanied by changes in blood pressure. In addition, transection of the vestibular nerves greatly impairs blood pressure compensation to head-up tilt in cats, suggesting that the vestibular system participates in correcting orthostatic hypotension. The existence of vestibulosympathetic reflexes which act on the cardiovascular system is not surprising, since changes in posture can challenge the maintenance of stable blood pressure, and the vestibular system provides inputs which signal when changes in body position, including those which can affect circulation, are occurring. Our long-term goal is to define and study the neural circuitry responsible for producing vestibulosympathetic reflexes. Our previous work has shown that neurons in two regions of the brainstem, the caudal medullary raphe nuclei and the subretrofacial portion of the rostral ventro-lateral medulla, transmit vestibular signals to sympathetic preganglionic neurons in the spinal cord. The proposed experiments are aimed at discovering which brainstem neurons participate in relaying labyrinthine inputs to these spinally-projecting cells. By making use of lesioning techniques and electrophysiological methods, we will determine: (1) which vestibular nuclei mediate vestibulosympathetic reflexes; (2) which brainstem interneurons relay vestibular signals to spinally- projecting vasomotor neurons; and (3) whether the cerebellum influences vestibular-elicited sympathetic responses. The proposed work will thus advance the basic scientific knowledge about an underinvestigated and potentially important neural mechanism involved in the control of circulation.